1. Field of the Invention
The present invention relates to a fluid pressure control apparatus, which controls a fluid pressure at a control subject chamber.
2. Description of Related Art
In the following description, “valve closing” refers to closing of an input port of a valve housing. Furthermore, “valve opening” refers to opening of the input port.
Japanese Unexamined Patent Publication No. 2006-046640 discloses an electromagnetic hydraulic pressure control valve as an example of the fluid pressure control apparatus.
The electromagnetic hydraulic pressure control valve, which is disclosed in Japanese Unexamined Patent Publication No. 2006-046640, will be described with reference to FIGS. 8A and 8B. In the following description, components similar to those of embodiments of the present invention, will be indicated by the same numerals.
The electromagnetic hydraulic pressure control valve 10 shown in FIG. 8A is of a normally low (N/L) type, i.e., a positive logic type or a normally closed type. The electromagnetic hydraulic pressure control valve 10 of the N/L type includes a spool valve 13 and an electromagnetic actuator 15. The spool valve 13 includes a spool 12, which is urged in a valve closing direction by a return spring 14. The electromagnetic actuator 15 drives the spool 12 in a valve opening direction against the urging force of the return spring 14.
In the electromagnetic hydraulic pressure control valve 10 of the N/L type, the return spring 14 serves as a valve closing direction drive means, and the electromagnetic actuator 15 serves as a valve opening direction drive means.
The electromagnetic hydraulic pressure control valve 10 shown in FIG. 8B is of a normally high (N/H) type, i.e., a negative logic type or a normally open type. The electromagnetic hydraulic pressure control valve 10 of the N/H type includes the spool valve 13 and the electromagnetic actuator 15. The spool valve 13 includes the spool 12, which is urged in the valve opening direction by the return spring 14. The electromagnetic actuator 15 drives the spool 12 in the valve closing direction against the urging force of the return spring 14.
In the electromagnetic hydraulic pressure control valve 10 of the N/H type, the return spring 14 serves as the valve opening direction drive means, and the electromagnetic actuator 15 serves as the valve closing direction drive means.
In both of the N/L type and the N/H type, notches 39 are formed in at least one of an input seal land 23 of the spool 12 and a valve housing 11 to open the input port 17 in a small amount.
In FIGS. 8A to 9C, although the notch 39 is provided in the input seal land 23 of the spool 12, the notch 39 may be provided in the valve housing 11 in some cases.
The disadvantages of the above prior art will be described with reference to the electromagnetic hydraulic pressure control valve 10 of the N/L type.
The spool 12 receives a force (hereinafter, referred to as a valve closing spring force), which is applied from the return spring 14, and a force (hereinafter, referred to as a feedback valve closing force), which is applied due to a pressure increase in a feedback chamber 27. Here, at the time of valve closing, the output hydraulic pressure is not generated in the feedback chamber 27. Furthermore, right after the valve opening, the hydraulic pressure of the control subject chamber is still increasing, so that the output hydraulic pressure is not substantially generated in the feedback chamber 27.
When the input seal land 23 is moved from the valve closing state to communicate between the input port 17 and the output chamber 25, the notch 39 is opened first. Thereby, as indicated by an arrow a in FIG. 9A, a high velocity oil flow passes through the notch 39 (the small gap between the input seal land 23 and the input port 17).
The flow direction, which is indicated by the arrow a in FIG. 9A, is the axial valve closing direction, so that the high velocity oil flow, which passes through the notches 39, provides a fluid reaction force (see an arrow β in FIG. 8A or 8B) that acts as a valve closing force on the spool 12 at a location X in FIG. 9A.
Furthermore, the high velocity oil flow, which is supplied into the output chamber 25 through the notches 39, impinges against an effluent seal land 24 of the spool 12 to provide a fluid reaction force (see the arrow β in FIG. 8A or 8B) that acts as the valve closing force on the spool 12 at a location Y in FIG. 9A.
As described above, right after the valve opening, the fluid reaction force (β) is applied to the spool 12 to push back the spool 12 in the valve closing direction, so that the pressure increase at the output port 18 is disadvantageously limited, and thereby the good response cannot be obtained.
Furthermore, in order to obtain the good response, the electromagnetic actuator 15 needs to generate a reaction force canceling drive force (see an arrow β′ in FIG. 8A), which overcomes the fluid reaction force (β), right after the valve opening.
Thus, the required valve opening drive force (see an arrow γ in FIG. 8A) of the electromagnetic actuator 15 is disadvantageously increased. Thereby, the size of the magnetic circuit of the electromagnetic actuator 15 needs to be increased, and the electric power saving of the electromagnetic actuator 15 is limited.
Particularly, in the recent years, the supply hydraulic pressure tends to be increased to cause an increase in the fluid reaction force (the valve closing force). Thus, the size of the electromagnetic actuator 15 tends to be increased, and the electric power consumption of the electromagnetic actuator 15 tends to be increased.
Here, although the disadvantages of the electromagnetic hydraulic pressure control valve 10 of the N/L type have been described, the electromagnetic hydraulic pressure control valve 10 of the N/H type also have the disadvantage of the applying the fluid reaction force (the valve closing force) to the spool 12 right after the valve opening.